A.2.b Close Reading Research

Step 2. Scientific Research is iterative: Today we are conducting close reading so we can integrate our research into what we already know and improve our working model.

What Road Salt Is and How It Works

Have you ever seen big trucks with plows on the road in the winter? You see them scoop up snow and push it off the road. You might not know or not have seen that the trucks also put salt on the road. Yet, you may have noticed the sand and brown colors in the snow. Oftentimes, the salt put on the roads is combined with sand. The sand helps cars with traction. But what exactly is the salt doing?

During cold weather, stores and cities keep big bags or piles of road salt to sprinkle on sidewalks and roads. Have you ever seen the blue-icy looking crystals that people put on sidewalks to keep people from slipping? Road salt is used to melt ice. But what is road salt and how does it work?

How Road Salt Works

Road salt is a crystalized mineral that breaks into smaller parts in liquids. When the mineral or road salt dissolves in water, it lowers the freezing point in a process called freezing point depression. The salt particles depresses or reduces the temperature at which water will freeze. The salt particles do this by getting in the way of the hydrogen bonds and preventing the water molecules from forming into solid crystals. Almost any substance that dissolves in liquid has this effect. For example, sugar and molasses can dissolve in water and be used as a molecule to lower the freezing point of water. However, salt’s low molecular weight gives it 6 times the effectiveness of sugar in freezing point depression.

Freezing point depression

The freezing point of a liquid is the temperature at which the liquid begins to turn to a solid. Freezing point depression means that the temperature at which water freezes is lowered. When salt is added to ice or snow, it has the effect of melting ice because the freezing point depresses lower than 32 degrees. For road salt to work there needs to be a tiny bit of liquid water. This is part of the reason why road salt is not effective in extremely cold weather when water would freeze too easily. In low freezing temperatures, between 25 degrees and 32 degrees there is usually enough liquid present from the heat caused by the friction of traffic.

Next time you see salt crystals on the sidewalk or big trucks sprinkling salt on the roads, you now know that they are lowering the freezing point of water or causing ice to melt.

Sources

Helmenstine, Anne Marie, Ph.D. "How Salt Melts Ice and Prevents Freezing." ThoughtCo, Aug. 26, 2020, thoughtco.com/how-salt-melts-ice-3976057.

1. Briefly summarize the main point of this article.

2. What happens to salt when it dissolves?

3. How does salt melt ice? Use the word Freezing Point Depression somewhere in your response.

4. The article does not tell you, but why do you think salt is 6 times more effective than sugar at lowering the freezing point of water?